The data is stored in ~/Library/Safari/CloudTabs.db which is an SQLite database that can be opened with the free tool DB Browser for SQLite
Once you have it opened you can dump out the URLS by going to the Browse Data tab, opening the cloud_tabs Table and copying out the URL column.
If you use OneTab You can even copy out the title field as well, dump it in Excel, put the URL field in column A, a | in column B and the Title in column C
copy that to the clipboard, dump it into your favorite text editor, do a find and replace on the | and replace the tabs with spaces,
then copy the whole thing into OneTab.
dump your list in there and then you can open them all at once or browse at your leisure.
I found an Alfred Workflow that automates this and edited it to dump the output to a text file on my desktop that I can then open and import into OneTab.
The main thing I changed was to output to text instead of markdown, and the format of the outtext at the end. Thanks to kmarchand for the code.
outtext += '%s | %s\n' % (tab['URL'], tab['Title'])
outtext += '\n'
Now I can just open the file and dump it into OneTab. Next step will be working out how to automate that.